1. Field of the Invention
This invention relates to a reverse buckling rupture disc assembly especially adapted for use in sanitary pressure vessel piping applications where the disc reliably ruptures under overpressure conditions.
Pharmaceutical, biochemical and food processing equipment require that sanitary conditions be maintained at all times, which necessitates frequent cleaning of the equipment, usually with steam or other sanitization agents. These processes often are operable at relatively low pressures in which overpressures in the equipment or piping connected thereto must be relieved at levels as low as about 2 psig up to about 50-60 psig. It is conventional to employ reverse buckling rupture discs for applications, but it has been found difficult to provide narrow range burst pressure tolerances at these ranges.
In order to accomplish reliable disc rupture at low differential pressures, it has now been found that a required rupture specification can be met while at the same time avoiding material collection problems on the surface of the disc by subjecting the disc material to a force which deflects a segment region of the disc from the main body thereof, and by thereafter applying a force to the disc which returns the deflected segment region to its initial position whereby the metal of the deflected and returned segment region has an altered grain structure as compared with the metal of the remainder of the central bulged section. The metal of the deflected and returned segment region exhibits higher residual stress, resulting from strain hardening through plastic deformation, than the disc material surrounding the initially deflected segment region.
This invention also relates to an improved process for preparing a smooth bulged surface reverse buckling rupture disc assembly which will open reliably at pressures within a range of, e.g., about psig 2 to about 50 psig, and that can be mounted in standard quick coupling fittings commonly employed in sanitary condition processing equipment without modification of the coupling structure.
2. Background of the Invention
In order to obtain repeatable low overpressure opening of rupture disc assemblies designed for low pressure applications, one commercial approach to the requirement has been to provide a reverse buckling rupture disc in which a depression is deliberately formed in the dome of the rupture disc. The depressed segment region in the domed area of the disc is strategically located in a position such that the domed part of the disc will fail first at the area of the depression. The disc thus reverses and opens at what has been described in the prior art as an overpressure less than a disc without a depressed segment region.
However, a depression in the process side convex surface of the bulged area of the disc presents a cavity that serves as a collection point for food, pharmaceuticals or the like. As a result, cleaning of the processing equipment with steam or the like is difficult and may require breakdown of the components in which the rupture disc is positioned in order to insure removal of material that may have collected in the depressed segment region.
Exemplary of a prior art rupture disc assembly having a depression in the convex surface of the disc is Cullinane, et al., U.S. Pat. No. 6,494,074, in which a pointed tool forced against the backed up convex surface of the bulged section of a disc forms an indentation in the disc at or near the apex of the domed shape. The shape, area and depth of the indentation may be selectively varied. In each instance though, the indentation in the convex surface of the bulged section of the disc presents a cavity which may collect material from the process operation that is protected from a predetermined overpressure by the disc mounted in a pipe fitting leading to the processing pressure vessel. Although Cullinane et al. suggest that the depth of indentation may be altered, but not eliminated, the patentees did not perceive that a smooth surface disc could be provided which avoids material collection problems in an indentation in the disc by forming a depression in the disc which is then returned to its initial smooth surface position, while at the same time meeting stringent burst specifications.
FIGS. 6-9 of Graham et al., U.S. Pat. No. 6,318,576, illustrate a hygienic quick breakdown and reconnection fitting conventionally used in pharmaceutical, biochemical and food processing operations, which is adapted to receive and retain a reverse bulging rupture disc assembly. The fitting includes two couplings having flanges which are retained in adjacent interconnected relationship by a quick release clamp ring.
Reverse buckling rupture discs are preferred for a variety of applications because a reverse buckling disc will open at a pressure near the bursting pressure of the disc without producing fatigue and failure which oftentime occurs with a forward acting disc when the disc is operated near its burst pressure for long periods of time. One theory of the sequence of operation of a non-knife blade reverse buckling rupture disc is explained in Mozley, U.S. Pat. No. 4,512,171, which is incorporated herein by reference thereto.
A commercially acceptable reverse buckling sanitary rupture disc should ideally meet current ASME BPE (Bioprocessing Equipment) and 3-A (milk and dairy) standards, which require the equipment to be free of surface imperfections such as crevices, gouges, obvious pits, etc.